[clang] [clang] more useful error message for decomposition declaration missing initializer (PR #127924)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

Thank you for improving the summary. @erichkeane mentioned that templates are a special case for the diagnostics with your original approach. Can we add a test that deal w/ templates to verify that the diagnostic works in those case properly? It still looks like you are relying on the preprocessor IIUC.

https://github.com/llvm/llvm-project/pull/127924